Friction differential gear



Jan. 4, 1927.

N. TRBOJEVICH FRICTION DIFFERENTIAL GEAR Filed June e, 1923 2sheets-sheet 1 mt, www www@ Patented Jan. 4, 1927.

UNITED STATES PATENT OFFICE.

NIKOLA TRBOJEVICH, OF DETROIT, MICHIGAN.

Application filed June 6, 1923. Serial No. 643,676.

'llie invention relates to a novel form ot a differential or epicyclicgear, one. that is particularly adapted for use in nal drives of motorvehicles. The dinverential now generally used in passenger cars andtrucks usually consists ot' a spider bolted to the ring gear, carrying anumber ot (usually tour) bevel pinions, and ot' two opposite bevel gearskeyed to right and lett wheel axles, engaging said planetary pinions atopposite sides. When both wheels receive an equal traction, thedifferential does not operate and the planetary pinions serve merely asdriving wedges or keys to propel the vehicle. However, it' for somereason or other one of the wheels is compelled to accelerate relativelyto the other, it is free to do so and the otherI wheel is automaticallyretarded a proportional amount.`

l have discovered that it is possible to greatly simplify the design otthe conven tional ,differential without sacrificing any of its operatingfeatures and, indeed, such a simplilication results in an improvedeiiiciency all around. Thus, my new differential, besides being simplerand easier to manufacture, is also more silent, more compact, morepowerful for its size, and easier to maintain in an operating conditionthan the conventional type.

Tn the drawings y Figure 1 shows a sectional View of a typicalconstruction embodying my invention;

Figure 2 is a modified construction having an added 'feature ofadjustability;

Figure 3 shows the plan view of the spider employed in the type shown inFigure 2;

Figures l and 5 are diagrams explaining the mechanical principlesinvolved;

Figure 6 shows another modi lied construction employing` barrel rollers.

Referring now to the Figure 1, the bevel pinion 11 engages the ring gear12 which is bolted to the spider 13 by number of Suitable bolts lel.Said spider is made preterably of some antitriction metal, suchl asbronze, and has a. number of smooth compartments 15 formed to house anumber of steel balls 16. The other end of said Spider 18 is termed intoa bearing r18a having a running tit with the shank 17"1 ot the innerrace 17.

The outer race 18 is a bell shaped forging,

hardened and accurately ground in its bearing surfaces. It co-operateswith the similarly linished inner 'race 17 in holding and compressingthe balls 16. In the central part ot said outer race 18 a hub 18a isformed the object otl which is to provide a suitable support for thebearing 17b of the inner race 17 and thus prevent the apparatus frombuckling under the radial thrust 'produced by the ring gear 12. Theraces 17 and 18 are however, rotatable with respect to each other, andthe friction created by said rotation is minimized by the employment ofthe bronze sleeve 19a as shown. The ends of the axles 2O and 21 aresplined and lit into thev corresponding carefully bored and splined hubsot the races 17 and 18. The whole apparatus is supported in the rearaxle housing by means of the two ball bearings 22v and 28, as indicated,

From the preceding description and the Y diagram Figure 4, the operationof the apparatus will be understood. The balls 1G are held andcompressed by the races 17 and 18 with considerable radial forces Q(Figure t). At the pointa therefore, there is a metal to metal contact,and a corresponding frictionforce F is generated which resist-s thetendency of the tangential torce T to slide the ball 16 on the tace ofthe ring 18. A similar condition obtains at the opposite point asituated on the circumference of the inner ring or race 17. It is clearfrom the diagram that if the sum of the two friction' forces F and F isgreater than T (the tangential load carried by each ball) the balls 16will not slip with respect to either ot the races 17 and 18 when thespider 13 is rotated, but will drag said races along and cause them torotate with the same angular velocity, and in the direction as indicatedby the arrows. Thus, the vehicle will be positively driven when bothwheels are geared to the road and are thus compelled to rotate in thesame direction. However, it one ot thc wheels is held tast (18 in Figure5) land the other 17) is permitted to rotate reely, and the spider Bpushes the balls 16 ahead as indicated by the arrow, the inner ring 17will rotate ahead (counter clockwise) with an increased velocity. Arotation` of the ring 18 in the lsan'ie direction will result it thering 17 is held fast. Furthermore, if the spider V13 is held tast andone of the wheels is' rotated, the other wheel will rotate in theopposite direction. It will be seen from this (soY that so long as thefriction forces F are being constantly generated, always in excess ofthe tangential load T, the mechanism as illustrated in Figure l, will befully equivalent to the conventional differential of the geared type andwill uniformly and positively drive both wheels ahead when they areevenly geared to the road but will permit a free differentiation at anyinstant between the two wheels if either one of the wheels is compelledby an outside force to increase or decrease its normal rate of rotation.lt ma/7 be stated, therefore, that the utility of this novel apparatushinges first on whether or notit is possible to constantly generate thefriction forces F of a suilicient magnitude to withstand the torquetransmitted by the ring gear, without slipping and with a sutlicientmargin of safety, and second, whether the wear of the members willrender the life of this appliance too short for practical pun poses. Asit is well known from mechanics the friction force, F27 Q, where f isthe coetlicient of friction, and Q is force of compression. According tothe experiments performed by Bennie and others, the value of saidcoeihcientof friction for slightly lubricated and smooth steel surfacesranges from 14- to l() percent, the higher values of f corresponding tohigher surface pressures. A simple calculation will show that by thisnew gear a considerable torque may be safely transmitted without`encoiintering any serious practical difficulty. t steel ball of only /qinch dian'ieter is able to sustain a. normal load of approximately 25000lbs. before being crushed. Now if we apply only one fifth of that loadto each ball, and further assume the value of f to be only l5 percent,it follows that lTzTO lbs. is there are two forces F for each ball, andsupposing that there are l5 balls employed, the total tangential forcetransmissible hy a mechanism Vof this type is over 22500 lbs., which isa strength `greatly in excess over that of a toothed wheel of a sim' arsize, and is suf- `licient to propel an ordinary n'iotor car. It will beapparent from this that a gear of this kind has a marked advantage over,the toothed gear of the conventional form. lVhile in an ordinary gearthe tangential load is carried usually by only a. few teeth at a time,in this case, all thel balls carry the load all the time, thus reducingthe load carried by each ball to only a small fraction of the totalload. Furthermore, as each ball generates two friction forces l?, theload element is still further halved for each point of contact.

A method of applying the compressing forces Q to the balls is thefollowing: rlhe balls 1G, all of which are of exactly the same diameter,are a little larger than the space between the two rings l? and 18 andare forced in between the ,two rings when the apparatus is assembled.Both the balls and the races are preferably made of alloy steel havinggreat tensile strength and surface hardness. @wing to the oversize ofthe balls, they are compressed to a slightly elliptical shape, and therings themselves are slightly deformed. However, all deformations innstbe kept within the elastic limits of the materials employed, as apermanent displacement would mar the surfaces and would make the properdifferentiation between the races l? and i8 impossible. lf thedimensions are properly selected it is possible to lreep the forces Qbetween two limits, that is, Q must be suiiiciently great to prog ducethe theoretically required friction forces l?, and yet it must be lessthan a certain maximum in order not to injure the halls and the races.

rthe effect of the wear in this mechanism will he to reduce tiediameters of the balls and increase the space between the rings. Thus,the maximum transmissible tangential force will gradually grow less andless as the gear wears out. However, owing to the fact that the ballswear only when they are rotating (as they cannot slin) and the rotationof the balls takes place only when the wheels are di'tferentiating, thatis, only occasionally and at. low speed, and further that the balls andraces are accurately ground and possess an extreme surface hardness; itwill be seen that a mechanism of this lrind will have very" little wearand will function for a long time without the need of any adjustment orattention.

Figure Q shows a slightly modified construction of my improveddifferential. rlhe worm 2li engages the worm wheel 25, the latter beingrotat' bly mounted on the bosses 2li and 27a formed at ends of the twowheel axles 26 and Q?. spider 28 (shown in plan view in Figure I3) islreyed concentrically to 'the Lrear 25 and carries a number of steelballs 29. 'lhe balls are compressed with a force exceeding the load tobe transmitted between the hardened and ground faces 2li" and 27') ofthe bosses Qttand 2?. 1 rlhe compression is obtained by tightening thenut SO engaging the threaded shank 3U of the axle 2h against the.thrustbearingl 3l which hearing agz'zin is mounted on and against theinner su fface of the hol- `e boss 27a. rl`he axles 26 and QT alsomutually support each other and thus prevent the apparatus fromhuclrling u ler the load through the iedium of the wide huh of the wormgear 25. fr bronze ler-.shing` 25h is inserted between the hub 25a ofthe gear and the outer circumference of the hollow axle boss 27a to fasiita-te assemhling and to reduce the friction which is generated when thewheels differentiate and the axles 2.6 and 27 rotate relatively to eachother. The operation of the apparatus is ill) fully analogous to that otthe previously described modification except that the compression torceQ in this case is not obtained by the initial springing ot the races,but by tightening the thrust nut. 30. I

Figure (i shows still another modification ot my improved differential.ihe ring 32 is bolted onto the spider 33 having a plurality ot smoothcompartments housing a number of barrel shaped rollers or balls 34, oneball in each compartment. The inner race is termed in the shape of abell-like sleeve having an accurately ground tapered hole 36 in the'liront end to tit the tapered adjusting plug 3? and a central bore 38the trout end ot which is threaded to tit the threaded shank 39 ot' saidadjusting plug, while the rear end 40 ot said bore is splined and fitsthe splined end ot the lett axle 4-1. The outer race a2 is also a bellshaped -torging ot comparatively massive proportions and isconcentrically bored and splined to receive the right axle 43. Theadjusting plug 37. as already stated, tits the tapered hole and thethreaded portion of the inner race S5 and is provided with a diametral`lreyway t4 in order that it may be tightened with a suitable wrench andthus cause the inner race 35 tol swell for expand. Said plug is alsoprevented trom working loose from its adjusted position in the race 35by means of one or more suitable keys or pins (not shown).

rIhe manufacture and assembly ot a mechanism ot this hind may be plannedand executed in many various ways, but the object is always tosubjectthe balls 34 to an intense radial pressure without injuring thesame or the races. Thus the outer ring l2 might be heated and shrunk onthe balls 34, and the taper plug 37 might be tightened at the same timeto increase the radial pressur still further. 'Ihe purpose ot the plug37 is not so much to provide adjust-ment after the apparatus has beenassembled and operated as it is to 'facilitate the manufacture andassembling ot said appliance. It will be understood that the presence ofthe taper plug 37 permits ot wider limits ot variations in the diametersof balls and races which tact will naturally tend to reduce the cost otproduction. A further advantage connected with the employment of saidadjusting plug lies in the tact that the elastic displacement ot themolecules in the inner race 35 is reduced as the plug 37 tends to expandthe race 35 outwardly While the forces Q tend to compress the sameinwardly. Under such conditions the resulting displacement is materiallyless than it the forces were acting in one direction only.

From the preceding description it will seen that the novelty of myinvention consists iirst ot my iin'ding a new use for a common ballbearing, and second in my discovering a new. principle regarding thedesirability ot. a friction gear in certain classes ot machinery.Because, prior to my discovery it was generally believed that a frictiongear is not capable ot transmitting heavy loads, and that the naturaliield tor t'riction gearing was in mechanisms running at a high rate ot'speed, and carrying but a slight load; I have partially reversed thatprinciple by maintaining that a friction gear ot the specitied kind maybe used tor transmission of even heavier loads than those which atoothed gear of approximately the same size would stand, although a highspeed and continuous running is detrimental on account ot wear. It willalso be readily understood that pins, rollers and tapered or barrelrollers may be employed instead ot rolls in mechanisms ot this kind andapproximately the same results obtained. However, in my preferredconstruction I employ tree steel balls only.

That I claim as my invention is l. In a diiferential, the combinationwith a driving member and a pair oi driven members, ot a spiderconcentrically mounted on said driving member, a plurality of rollingmembers carried by said spider, a pair ot races respectively carried bysaid driven members, and means for adjustably compressing said rollingmembers between said races with suiiicient torce to frictionallytransmit the torquetrom said driving member to said driven members.

2. In a differential, the combination with a driving member and a pairot driven members, ot a spider concentrically mounted on said drivingmember and provided with a plurality ot compartments, a plurality otballs carried by said spider in said compartments, a pair ot racesrespectively carried by said driven members, and means for adjustablycompressing the balls between said races with suiiicient torce tofrictionally transmit the torque from said driving member to said drivenmembers.

3. In a 'differential the combination ot a driving spider having aplurality of smoothly finished compartments, a plurality ot free rollingmembers concentrically disposed about the axis ot said spider androtatable in said compartments, an outer hollow-shellshaped drivenmember clastically compressing the rolling members on their outer'sides, an inner driven member compressing said members on their innersides, and means for expanding the inner driven member against therolling members.

4. In a ditlerential the combination of a 'driving spider having aplurality of concentric compartments, a plurality ot tree rollingmembers rotatable in said compartments, an outer elastic shell, an innerdriven member concentric with said spider and shell, a tapered plugfitting into said inner member, means for forcing and holding said plugin position tol radially expand the inner member.

5. In a diferential the combination of a driving spider having aplurality of Concentric compartments7 a plurality of free rollingmembers rotatable in said Compart ments, two driven members coaxial withthe spider and touching the rolling members on their opposite. sides,and means for compressing and holding the driven members against therolling members and each other to produce the required high pressureupon the rolling members and to permit of a free rotation of one drivenshaft relative to the other.

In testimony whereof I aiiiX my signature.

NIKOLA TRBOJEVICH.

